improved permeation performance and fouling-resistance of...
TRANSCRIPT
Improved permeation performance and fouling-resistance of
Poly(vinyl chloride)/Polycarbonate blend membrane
with added Pluronic F127
Supateekan Pacharasakoolchai and Watchanida Chinpa*
Department of Materials Science and Technology, Faculty of Science,
Prince of Songkla University, Hat-Yai, Songkhla, Thailand 90112
* To whom correspondence should be addressed. Phone: +(66) 74 288397.
Fax: +(66) 74 446925. E-mail: [email protected], [email protected]
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Original Article
Improved permeation performance and fouling-resistance of
Poly(vinyl chloride)/Polycarbonate blend membrane with added Pluronic F127
Received 16 Nov. 2012 Accepted 9 Dec. 2013
Abstract
The aim of this work was to prepare and characterize poly(vinyl chloride)
(PVC)/polycarbonate (PC) blend membranes for use in ultrafiltration. Pluronic F127
was used as an additive to modify the membrane surface of the PVC/PC blended
membranes. The PVC/PC blend membrane was first prepared using the phase inversion
method from a casting solution of PVC with small amount of PC in N-
methylpyrrolidone (NMP) and water as the non-solvent. The morphologies structure
and properties, such as tensile strength, water flux, and bovine serum albumin (BSA)
rejection of the blend membrane were studied. Increased amounts of PC resulted in an
increase in the water flux and ability to reject protein. A concentration of 0.75 wt% PC
provided the best improvement in tensile strength of blend membrane. Addition of
different amounts of pluronic F127 to the casting solution of PVC/PC with a PC
concentration of 0.75 wt% resulted in a decrease in the water contact angle that
demonstrated the improvement of hydrophilicity of blend membrane. Scanning electron
microscopy photographs showed that the modified PVC/PC membranes had a bigger
pore volume in the porous sub-layer compared to the PVC/PC control membrane. The
PVC/PC membrane with added Pluronic F127 exhibited a much higher flux and
rejection of BSA in a protein filtration experiment than the PVC/PC membrane. An
increase in flux recovery ratio of PVC/PC/pluronic 127 blend membrane indicated that
the modified membranes could reduce membrane fouling useful for ultrafiltration.
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Keywords:Poly(vinyl chloride), Polycarbonate, Blend membrane, Pluronic F127,
Ultrafiltration
1. Introduction
Phase inversion is the most commercially available method for preparing
microporous polymeric membranes (Strathman and Kock, 1977; Bottino et al., 1991). In
this method, a homogeneous polymer solution is cast onto a suitable support and
immersed in a coagulation bath containing a non-solvent, usually water. The exchange
of solvent and non-solvent induces the separation of the system into two phases: a
polymer-lean and a polymer-rich phase. The polymer-lean phase disperses into the
concentrated polymer-rich phase resulting in the formation of a porous structure
(Machado et al., 1999). Many polymers such as poly(vinylidene fluoride) (PVDF),
polysulfone (PSf), poly (ether imide) (PEI), polyacrylonitrile (PAN), and cellulose
acetate (CA) can form microporous membranes using this method due to their thermal
stability and chemical resistance (Kesting, 1971). However, these materials are
expensive to prepare.
Poly(vinyl chloride) (PVC) makes an interesting polymer used to fabricate
microporous membrane due to its stiffness, its excellent resistance to abrasion, acids,
alkaline and microbial corrosion, and it is a low cost product. Some research has
focused on the preparation of flat PVC ultrafiltration and microfiltration membranes
using phase inversion (Hirose et al.,1979; Hirose and Yasukawa, 1981; Bodzek and
Konieczny, 1991; Hiroshi et al., 1993; Peng and Sui, 2006; Chinpa, 2008). PVC
membranes for use in the ultrafiltration and microfiltration PVC for water treatment
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systems have been prepared by Gao et al. (1988). However, the drawback of pure PVC
is its brittleness and liability to fracture. A new material formed by blending PVC with
other polymers may have more desirable properties. A suitable polymer for blending
with PVC as the major component might be polycarbonate (PC) because it has a rigid
backbone with good mechanical strength and is suitable for producing a thermally
resistant membrane (Vijayalakshmiet al., 2008).
Due to the hydrophobic nature of PVC and PC, fouling on the membrane surface
could occur during filtration of solutions containing substances like protein. A blend
with a hydrophilic polymer material that has both hydrophilic and hydrophobic polymer
segments might have the advantage of providing opportunities for easy surface
modifications. Polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-
PEO) triblock copolymers have been introduced to blend with polyethersulfone (PES)
(Wang et al., 2006; Zhao et al., 2008) and cellulose acetate (CA) (Lvet al., 2007) to
reduce protein adsorption. The hydrophilic unit (PEO) not only improves permeation
properties and fouling resistance, but the hydrophobic part (PPO) could anchor with the
polymer membrane matrix (Wang et al., 2006; Zhao et al., 2008). Therefore, this
additive will remain with the membrane matrix during the filtration process (Ma et al.,
2004).
The aim of this work was to prepare a PVC membrane with a high
hydrophilicity, high water flux and good mechanical strength. In this work, the PVC/PC
blend membranes were first prepared using the phase inversion technique by adding a
small amount of PC into the PVC casting solution. The effects of the PC content on
membrane morphology, pure water flux, BSA rejection, and tensile properties were
studied. Secondly, Pluronic F127 was used as an additive to the casting solution to
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increase the hydrophilicity of the PVC/PC membrane. The effects of the Pluronic F127
content on membrane performance, morphologies, hydrophilicity, and fouling were
studied in detail and discussed.
2. Experimental details
2.1 Materials
PVC with a molecular weight of 48,000 g/mol was purchased from Fluka. PC is
a commercial product (PC hollow profile sheet) with a molecular weight of about
55,500 g/ mol analyzed by GPC using a PS standard running with THF at 40°C. The PC
was dissolved in chloroform and then precipitated in methanol in order to remove
substances added during its manufacturing process. Pluronic F127 (ethylene
oxide/propylene oxide block copolymer) from Sigma-Aldrich was used as a hydrophilic
additive. Anhydrous NMP was also purchased from Aldrich. NMP was used as a
solvent without any further purification. BSA with an average molecular weight of
67,000 Da was supplied from Fluka.
2.2 Preparation of blend solution
The blended solutions based on PVC, PC, and Pluronic F127, were prepared by
dissolving the three polymers in NMP in different proportions (Table 1) at 60-65°C.
The homogeneous solutions obtained were used within a week.
2.3 Preparation of membranes
The membranes were prepared by the classical phase inversion method using
water as the coagulant. PVC/PC or PVC/PC/Pluronic F127 blended solutions were cast
on a glass plate with a casting rod of 250 m thickness. The cast films were pre-
evaporated for 30 s in air (27+2°C, RH 75+2%) then immersed in a water bath for
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complete precipitation. The membranes were removed from the glass plate and washed
thoroughly with a large amount of distillated water. The membranes were stored in
distillated water until used.
2.4 Hydrophilicity of membrane surface
The hydrophilic property of the membrane surfaces was characterized by
measuring the water contact angle (WCA). A water droplet of a constant volume was
placed on the surface of the membrane using an automatic interfacial tensiometer (Data
physic).
2.5 Water absorbance
The water absorbance was also studied as reported by Zhu et al. (2007). The
water absorbance ratio (WA) was calculated according to
WA = [(wwet-wdry)/wwet] × 100 (1)
Where wwet and wdry represent the weights of dried membrane and membrane
soaked in water at room temperature for 24 hrs, respectively. Both measurements were
carried out three times and the average values have been reported.
2.6 Membrane morphology observations
The cross-sectional morphologies of the obtained membranes were characterized
by SEM (JSM 5200). The membranes were cut into pieces of various sizes. These
pieces were immersed for 30-60 seconds in liquid nitrogen and then broken. The
fractured membranes were coated with gold for initiating electrical conductivity before
analysis.
2.7 Tensile properties of blend membrane
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The tensile strength and elongation at break for various membranes were tested
by the Universal Testing Machine (LLOYD). The measurement was carried out at room
temperature and a pull rate of 5 mm/min.
2.8 Transport properties of the blend membrane
In order to measure the transport and separation properties of the blend
membranes, the UF experiments were carried out using a stirred dead-end filtration cell.
All experiments were conducted at room temperature using an operating pressure of 1
bar. The pure water flux was calculated as follow:
Jw = Q/AT (2)
where Jw is the pure water flux (Lm-2
h-1
), Q is the quantity of collected permeate
(L), at the sampling time (hour) and A is the membrane area (m2).
The solute rejection (R) of the blend membranes was tested with 0.1 g/dL of
BSA. The protein concentration was measured by spectrophotometer (UV-model
Lamda 25, Perkin Elmer) at a wavelength of 280 nm. The rejection of BSA was
calculated using the following equation:
R = (1-Cp/Cf)×100 (3)
where Cp and Cf are the concentrations in the permeate and the feed,
respectively.
The anti-fouling properties of the membrane were investigated as described
elsewhere (Zhu et al., 2007; Ma et al., 2007; Zhao et al., 2008; Arthanareeswaranet al.,
2009). The steady state pure water flux (Jw1) of the prepared membranes was first
measured. Then the feed solution of 1g/L of BSA solution (in phosphate buffer saline
with pH = 7.4) was introduced and the filtrate was collected at 0.1 MPa until a steady
state of flux (Jp) was attained. Subsequently, the membrane was washed with distilled
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water on a vibrator for 2hrs, and then the water flux was reevaluated and the steady-
state value was defined as Jw2. The flux recovery ratio (FRR) was calculated according
to the following equation (Ma et al., 2007; Zhao et al., 2008;Zhu et al., 2008):
FRR (%) = (Jw2/Jw1) × 100 (4)
3. Results and Discussions
3.1 Effect of the PC concentration on the morphologies, pure water flux, and BSA
rejection of PVC/PC blend membranes
PVC/PC asymmetrical membranes with different blend compositions were
prepared by phase inversion, immersion method. The SEM images of cross-sections of
PVC and the PVC/PC blend membranes (Figure1) showed that the pure PVC and the
PVC/PC blend membrane have an asymmetrical structure consisting of two skin layers
(top and bottom skin layer) and a finger-like support layer or macrovoid. Generally, this
structure is obtained when the casting solution is immersed directly into a non-solvent
bath (van de Witte et al., 1996). As shown in Figure 1(a), for a pure PVC membrane, a
thick wall of finger-like pores is formed. As shown in Figure 1(b)-(d), finger-like pores
with the thinner wall formed when PC was added to the PVC casting solution.
Membranes with this structure might result in a higher pure water flux. In the case of
the membrane blended with 3 wt% PC, most of the finger-like pores extended to the
bottom of the membranes. It can be deduced that with an immiscible system such as
PVC/PC (Neill and Karasz, 2000) a decrease in the interfacial adhesion could take place
because of an increase in the dispersed phase content (Wu et. al., 2006). Following Wu
et al. (2006) it could be explained that when the PC content is higher than 1.5 wt%, the
larger volume of the PC dispersed phase would form leading to a wider distance
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between the PVC and PC phases. The finger-like pores would be formed under the skin
layer and grow towards the bottom of the membrane.
The flux of pure water and rejection of BSA by membranes prepared from the
casting solution with different composition are shown in Fig.2. The addition of PC to
the PVC greatly improves the water flux of PVC. The pure water flux of the PVC/PC
blend membrane increased from 9.7 to 147 L/m2-h with the increase of the PC
concentration from 0 to 3 wt%. The increase in the water flux is consistent with the
structures observed in the SEM photographs.
3.2 Effect of PC concentration on tensile properties
During the filtration process, an operating pressure is necessary to force the
filtration substance through the porous membrane. The mechanical properties (tensile
strengths and elongation at break of the membranes) were then studied and results are
shown in Figure 3. The tensile strength (TS) slightly increased when PC was added to
the PVC casting solution at a concentration of 0.75 wt%, and then TS decreased with
the further increase of PC amount. The elongation of membranes was lower as an
increase in amount of PC. A decrease in tensile properties might be due to the lack of
interaction between the PVC and PC.
Membranes prepared from a casting solution with a PC content of 0.75 wt% had
the highest mechanical strength, a higher pure water flux and BSA rejection compared
with pure PVC membrane, the effects of adding different amounts of Pluronic F127 to
this casting solution on the membrane performance were investigated .
3.3 Effect of Pluronic F127 on the hydrophilicity of a PVC/PC blend membranes
It is known that in the ultrafiltration process adsorption and deposition of protein
on the membrane surface or in pores causes fouling (Huimanet al., 2000; Ma et al.,
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2000; Feng et al., 2005). An increase in the membrane surface hydrophilicity can reduce
membrane fouling. Several workers have used the water contact angle (WCA)
measurement to study the hydrophilicity of membranes (Peng and Sui, 2006; Wang et
al., 2006; Zhao et al., 2008).
Figure 4 shows the WCA as a function of the Pluronic F127 content. The
hydrophobic character of the PVC/PC membrane (PC concentration of 0.75 wt %) is
demonstrated by the high WCA (ca. 84°). Adding Pluronic F127 causes a decrease in
the WCA to about 63°. These results indicated that the hydrophilicity of PVC
membrane was greatly improved. The improvement of hydrophilicity could be
attributed to the segregation of hydrophilic PEO units of Pluronic F127 on the
membrane during membrane forming process, while hydrophobic PPO segments were
trapped into PVC/PC membrane matrix (Wang et al., 2006; Zhao et al., 2008; Chen, et
al., 2009). This improvement of hydrophilicity implied that these membranes would
have a higher permeability in the filtration process (Yu et al., 2009).
3.4 Efect of Pluronic F127 on morphology, water content and transport properties of
the blend membranes
Figure 5 shows the SEM photographs of membranes prepared from the casting
solution with different amounts of Pluronic F127. The blend membrane from casting the
solution without pluronic F127 had a thick skin layer with a small size of the marcovoid
wall (Figure 5(a)). In the presence of Pluronic F127 as shown in Figure 5(b)-(d),
membranes had a thinner skin layer and larger macrovoid size. Membrane with this
structure might provide a smaller resistance resulting in a higher pure water flux (Wu et
al., 2006). A similar finding was reported for CA/Pluronic F127 membranes (Lv et al.,
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2007). The addition of Pluronic F127 into the casting solution results in a bigger volume
of the marcovoid sub-layer due to leaching out of Pluronic F127 to the coagulant.
Measurements of the water content of the PVC control and PVC blend
membranes showed that adding Pluronic F127 produced drastic changes in the water
content of the blend membranes (Fig.4). WA increased from ca 4% to ca 82%. This can
be explained by Pluronic F127 increasing the pore volume under the skin layer of the
membrane (Fig.5). These pores could then fill with water (Lv et al., 2007).
The variations in the pure water flux and rejection of BSA by the casting
solution based on PVC/PC polymer at a PC concentration of 0.75 wt% containing
Pluronic F127 at concentrations of 0, 3, 6 and 9 wt% are shown in Figure 6. Adding
Pluroic F127 to the casting solution produced a much higher pure water flux and
rejection of BSA rejection compared with PVC/PC membrane. The water flux value
was only 21.5 L/m2h for PVC/PC membrane and increased significantly to 506 L/m
2h
with a Pluronic F127 content of 3 wt%. The increase in pure water flux could be
explained by considering the SEM photographs. However, it was found that at a
concentration of Pluronic F127 of 9 wt%, the pure water flux dramatically decreased. It
was deduced that an increase in Pluronic F127 content to 9% might increase the
viscosity of the casting solution and lead to thicker skin layer of the membrane formed
by delayed phase inversion; see Figure 5 (e)-(f).
3.5 The effect of Pluronic F127 on anti-fouling properties of blend membrane
In order to study anti-fouling properties of the blended membranes, FRR was
evaluated by using BSA as the protein model. The higher the FRR values the better are
the anti-fouling properties of the membrane (Wang et al., 2006). The values of FRR for
all blend membranes increased with the presence of Pluronic F127 (Figure 7). This was
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due to the introduction of a hydrophilic PEO layer at the membrane surface, which
agrees with water contact angle data. These results implied that casting a solution with
Pluronic F127 provides for a reduction of the fouling and an improved flux recovery.
4. Conclusion
PC/PVC blend membranes were prepared simply by the immersion precipitation
method. The blend membrane with a PC concentration of 0.75 wt% offered a higher
pure water flux and higher mechanical strength when compared with a pure PVC
membrane. By adding pluronic F127 to the PVC/PC based casting solution, the
hydrophilicity of the membranes was significantly increased. In addition, membranes
with a larger macrovoid and a thinner skin layer were obtained that resulted in an
enhancement of the pure water flux. An increase in the FRR of the membrane indicated
that the PVC/PC based membranes containing Pluronic F127 could reduce the
membrane fouling and improve their usefulness for applications for ultrafiltration.
Acknowledgement
The authors are grateful to the Department of Materials Science and Technology,
Faculty of Science, Prince of Songkla University, for financial support. The authors also
acknowledge B. Hodgson’s assistance with the English language.
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Fig.1
Fig.2
(a) (b)
(c) (d)
17
Fig.3
Fig.4
18
Fig.5
19
Fig.6
Fig.7
20
Figure Captions
Fig.1. Cross-sectional SEM morphology of PVC (a), PVC-PC1 (b), PVC-PC2 (c), and
PVC-PC3 (d) membrane.
Fig.2. Pure water flux and BSA rejection of PVC/PC membrane as a function of PC
content.
Fig.3. Mechanical properties of PVC/PC membrane as a function of PC content.
Fig.4. Water contact angle and water absorbance ratio of PVC/PC membrane as a
function of Pluronic F127 content.
Fig.5. Cross-sectional SEM morphology of PVC-PC1 (a), PVC-PC1-Plu3 (b), PVC-
PC1-Plu6 (c), and PVC-PC1-Plu9 (d) membrane. Magnification of upper layer
of PVC-PC1-Plu3 (e), and PVC-PC1-Plu9 (f).
Fig.6. Pure water flux and BSA rejection of PVC/PC/Pluronic F127 membrane as a
function of Pluronic F127 content.
Fig.7. Flux recovery ratio as a function of Pluronic F127.
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Table 1 Casting solution composition for preparing blend membrane.
Membrane Casting solution composition PC
(wt%)
Pluronic F127
(wt%) PVC
(g)
PC
(g)
Pluronic
F127(g)
NMP
(g)
PVC 3.00 - - 17 - -
PVC-PC1 2.85 0.15 - 17 0.75 -
PVC-PC2 2.70 0.30 - 17 1.5 -
PVC-PC3 2.40 0.60 - 17 3.0 -
PVC-PC1-Plu3 2.85 0.15 0.6 16.4 0.75 3
PVC-PC1-Plu6 2.85 0.15 1.2 15.8 0.75 6
PVC-PC1-Plu9 2.85 0.15 1.8 15.2 0.75 9